Network operators upgrading their networks from 2G/3G networks to 4G networks want a network evolution solution which permits the leveraging of a portion of their installed baseband equipment. In some cases this means that multi-standard mixed mode radios are used to support efficient in-band combining to ease the service transition from 2G/3G equipment to 4G. Emerging deployments of shared radio solutions for base stations operating on multiple standards face particular challenges in relation to operating with clock synchronization. Each standard usually runs in its own clocking domain.
When the timing or synchronization reference is temporarily lost, a network's ability to maintain time and sync stability becomes critical to ensure continued optimal network performance. The time period that a network is able to maintain time and sync stability without a reference is called the holdover time.
Precise synchronization is especially critical in mobile networks for the successful call signal handoff and proper transmission between base stations, as well as for the transport of real-time services. If individual base stations drift outside the specified frequencies or time alignment, mobile handoff performance decays, calls interfere, and calls cannot be made, resulting in high dropped-call rates and impaired data services. In the event that timing or synchronization reference is temporarily lost, a network's ability to maintain time or “holdover” becomes critical to ensure optimal network performance.
These synchronization issues are more significant in shared baseband deployments since holdover requirements are not standard; that is, they vary depending on the system type, complexity, and operator's requirements. In shared baseband deployments, two heterogeneous systems with independent synchronization inputs are expected to produce outputs which are synchronized to one another, typically so that they can be combined together without loss of data. Existing clock synchronization solutions provide limited holdover capabilities in the case where one or both of the systems lose their synchronization inputs, leading to the outputs becoming unsynchronized with each other. Once the clock of one system is synchronized with the clock of the shared system, switching to a single clock can be initiated. However, the system must be able to switch the radio reference clock between a primary clock and a secondary clock in the event one or the other fails. Recovery or switching back to the main or primary clock should only occur if the clock is considered to have a stable source.
For these reasons, traditional synchronization and clock switching techniques have limited capabilities in situations as described above.